A gear, or other workpiece, may be inductively heat treated by passing an ac current through an induction coil. The current creates a magnetic field around the coil that magnetically couples with the gear to induce eddy current in the gear. Induction hardening of gears provides a martensitic layer at the tooth surface of the gear to increase hardness and wear resistance while allowing the remainder of the gear to be unaffected by the process. The increase in hardness also improves contact fatigue strength and other mechanical properties. The geometrical complexity of gears and variation in electromagnetic coupling between the induction coil, and tooth tip and root fillet, results in different induced heat intensities in the tip versus the root of the gear.
Frequency of the current makes a noticeable effect on eddy current flow within the gear and heat distribution. Basically when it is necessary to harden the tooth tips only with a single frequency of current using a single-turn or multi-turn solenoid coil, a relatively high frequency (e.g. 30 kHz to 450 kHz) and high power density are applied. See for example FIG. 1(a). When relatively high frequency current (power) is applied to coil 100, eddy current induced heating in gear 102 follows the contour of the gear as indicated by representative heating profile lines 104. Since the highest concentration of the current density will be in the tip of tooth 106, there will be a power surplus in the tip compared to root 108. Taking also into account that the tip of the tooth has the minimum amount of metal to be heated compared to root 108, the tip will experience the most intensive temperature rise over the entire heating cycle. In addition, from the thermal perspective, the amount of metal beneath the gear root represents a much greater heat sink compared to the tooth tip. Another factor that also complements the more intensive heating of the tooth tip deals with a better electromagnetic coupling due to the electromagnetic proximity effect between the inductor coil and tooth tip in comparison to the root; higher frequency has a tendency to make the proximity effect more pronounced.
When inductively hardening tooth root 108, a relatively low frequency (e.g., 50 Hz to 20 kHz) is preferable. With a low frequency, the eddy current penetration depth is much greater than with high frequency. When heating fine pitch and medium pitch gears it is much easier for low frequency induced current to make a short path and follow the base circle or root line of the gear instead of following the tooth profile. See for example FIG. 1(b) and representative heating profile lines 110. The result is more intensive heating of the root fillet area compared to the tip of the tooth.
Typically, in order to provide a hardness pattern that follows the profile of the gear tooth (from tip to root) preheating of the gear is required. Depending upon the gear geometry, preheating is usually accomplished by using a medium or low frequency (e.g. less than 20 kHz). High frequency (e.g. 30 kHz through 450 kHz) is applied during the final heating stage.
FIG. 2 illustrates one prior art method of inductive heating that utilizes a single coil 114 and two inverters 116a and 116b that are sources of low (or medium) frequency power, at low power density, and high frequency power, at high power density, respectively. The salient steps of the method are: place a gear within coil 114; rotate the gear; apply low frequency current from inverter 116a (by opening contacts 118 and closing contacts 120) to the coil to inductively preheat the gear; disconnect the coil from inverter 116a (by closing contacts 118) and apply high frequency current from inverter 116b (by opening contacts 120) to heat the gear to hardening temperature; remove the high frequency current from the gear; and quench the gear. Major drawbacks of this method is the system's low reliability and high cost. High currents are required for induction heating, and high current electromechanical contacts usually have a short life. Longer lasting electronic switches can be used in lieu of mechanical switches, but this would increase the overall cost of the system.
FIG. 3 illustrates another prior art method of inductive heating that utilizes two coils, namely preheat coil 128a and final heat coil 128b, and two inverters 130a and 130b. Medium frequency power inverter 130a supplies power to the preheat coil at low power density and high frequency power inverter 130b supplies power to the final heat coil at high power density. In this method gear 102 is sequenced by a suitable mechanical transfer system (in direction of arrow shown in FIG. 3) through preheat coil 128a, final heat coil 128b and quench ring 132 to harden gear 102 (shown in preheat coil). A major drawback of this method is that a short time between preheating and final heating (e.g. less than 1 second) may be required. This results in increased equipment cost, since the mechanical transfer system must be of a precision design for fast (fraction of a second) and accurate gear transfer from the preheating position to the final heat position.
In another prior art method simultaneous dual frequency power supply is used for gear hardening, for example, as disclosed in U.S. Pat. No. 2,444,259 (Method of High Frequency Induction Heating). The output of the simultaneous dual frequency power supplies consists of two appreciably different frequencies. One of frequencies provides heating of the root fillet and the other frequency provides heating of the tooth contour. A major drawback of this simultaneous dual frequency heating method is that the shape of the single coil cannot be optimized for both frequencies.
One objective of the present invention is improving the uniformity of induction hardening of gears (in particular, but not limited to, conical gears and pinion gears) and the reduction of gear distortion by using a relatively low frequency C-core type induction heating of the workpiece in combination with a relatively high frequency of induction heating of the workpiece using a separate induction coil.